It wasn’t too many years ago when most recipes that referred to adding hops usually only mentioned the variety of hops used, the amount, and not much else. This system worked relatively well at a time when hops were available in a few limited varieties and the freshness and quality were often questionable. But as the hobby evolved and became more sophisticated, a better, more precise way to calculate potential bitterness in a homebrew recipe was needed.
One of the simplest ways to more accurately determine bitterness levels and hop usage centers on the International Bitterness Unit (IBU). An IBU can technically be defined as one milligram of iso-alpha acid per liter of beer, which also equals one part per million (ppm).
Exactly what does this mean to us as homebrewers? Most published guidelines for various beer styles quantify the appropriate bitterness or hop level for a particular style in terms of IBUs. These IBU figures can range from as low as six for a classic American pilsner style to 60 or higher for a more bitter beer such as an India pale ale.
Sometimes in brewing literature IBUs are referred to simply as Bitterness Units (BU), but the two terms are the same and are interchangeable. For the sake of simplicity the term IBU will be used here.
It is important not to confuse IBUs with another common hop bitterness measurement used in homebrewing, the Alpha Acid Unit (AAU), also called the Homebrew Bitterness Unit (HBU). AAUs/HBUs are the bitterness potential of hops based on the percent alpha acid.
The AAU/HBU measurement of a hop addition is determined by multiplying the amount of hops by the hops’ alpha acid. While AAU/HBU formulas are quite simple and are still used in homebrewing, they have limited value in determining overall bitterness because they don’t factor in length of boil and other important parameters.
IBU calculations are just as simple, much more accurate, and are adaptable to a wider range of brewing situations. However, to be able to meaningfully incorporate IBU figures into brewing, you need to know and understand the basic IBU formula: IBU = Hops x AA% x utilization volume x 1.34
- Hops = the weight of hops in ounces
- AA% = alpha acid percent
- Utilization = the utilization percent
- Volume = the volume of the final batch in gallons
- 1.34 = a constant to convert measurement into US standards
There are many different published IBU formulas, most of which are mathematically very close if not the same. Regardless of which formula you use, the most important things to understand to successfully integrate IBU calculations into your homebrewing are the elements in the formula and how the elements relate to each other.
Because one IBU is defined in terms of a metric measurement of one milligram of iso-alpha acid per liter of beer, the constant of 1.34 is used regardless of the other figures in the formula. This automatically converts the results into the standard US measurements of ounces and gallons. For homebrewers who prefer to measure in terms of grams of hops in liters of beer, substituting a constant of 10 ensures the results will be accurate in metric terms.
Finished Batch Volume
For those brewers who are able to boil only a portion of their wort and then must top up with cold water to get to the final volume in the fermenter, it is important to use the final volume and not just the volume boiled with the hops.
The same is true for brewers with monster brewpots who boil a higher volume of wort than will be transferred to the fermenter. This is where IBUs have a big advantage over other hop calculation formulas such as the AAU/HBU. The volume of the batch is actually built into the equation to help calculate the amount of hops to use, making it easy to convert recipes between batches of different sizes.
A few years ago the standard batch of homebrew was five gallons, but today it is not uncommon to see batch sizes of 2.5, 10, and 15 gallons. With the batch size built into the formula, a 30 IBU Dortmunder Export should have the same relative level of bitterness whether it was brewed as five gallons or 50 gallons.
Many of today’s commercial craft brewers include IBUs on bottles or six-pack carriers, allowing the homebrewer to research different flavor and bitterness profiles long before filling their own brewpots with their latest creations.
Weight of Hops Used in Ounces
Most hops are available in premeasured packets of one or two ounces. For brewers using bulk hops, a small scale capable of accurately measuring increments down to one-quarter of an ounce is a must.
Alpha Acid Percent
Alpha acids are those compounds within the hop cone that provide the bittering. The alpha acid value that appears on the package indicates the portion of the weight of the hops that is made up of these bittering compounds, usually expressed as a percentage. To obtain accurate results when doing IBU calculations, use only fresh hops, well packaged in oxygen-barrier packaging and properly stored in refrigerated conditions.
Because the alpha acid value that appears on most hop labels is an average value that was attached to the larger package from which the hops originally came, it is important to try to purchase a product that has been packaged and stored to minimize the effects of oxidation and deterioration. This is crucial particularly if you are using whole or leaf hops instead of pellets. If there is no alpha acid value on the label, avoid the package of hops altogether and look for a more reliable product.
This portion of the formula makes it easy to switch among hops of different alpha ratings and helps ensure that a batch of homebrew made with this year’s hops will have the same relative bitterness as a batch made last year using a different crop with a different alpha acid rating.
Utilization Percentage — The Key to Consistency
The final variable in the IBU formula is utilization percentage. In a perfect world it would be nice to assume that all of the potential bitterness available from the hops is extracted into the finished beer, but unfortunately that is not the case. Many factors affect the amount of bitterness actually liberated, or isomerized, into the finished product. Some of the more common factors:
Length of Boil: The longer the hops are boiled, the more bitterness is extracted into the finished beer. This is generally good up to about 90 minutes, at which point the law of diminishing returns takes effect and less bitterness is extracted per unit of time. Conversely, late hop additions for flavor or aroma will contribute considerably less bitterness due to the reduced amount of boil time.
Intensity of Boil: Homebrewers using a mega-BTU outdoor cooker can virtually blast the enamel off a steel pot and will have a much more vigorous or intense boil than the homebrewer using an electric range. The more intense and turbulent the boil, the more hop bitterness is extracted.
Volume of Boil: The greater the volume of wort being boiled, the more effective the extraction of hop bitterness will be. Don’t confuse this with final wort volume in the IBU formula. For brewers with smaller brewpots who can only do a partial wort boil, it is important to realize that the reduced volume of the boil will reduce the extraction of the hops.
Specific Gravity of Wort: Dense, high-gravity worts with lots of dissolved sugars will reduce the ability of the wort to extract alpha acids in the boil. The purchase of a large-volume brewpot (seven gallons or greater) and the attendant accessories such as a high-output burner is even more appealing when you consider that a reduced wort volume combined with the increased gravity associated with only boiling a portion of the batch and then later topping off to final volume is a double whammy against effective hop bitterness extraction.
Quality of Hops: Old or stale hops will lose some of their alpha acid value. Also, pellet hops usually provide a slightly greater degree of utilization in the boil than their whole leaf counterparts of equal alpha acid value.
Fermentation: The use of a small-volume fermenter and a blow-off tube for the early stages of fermentation will result in a portion of the hop resins being "blown off" with the foam from the kraeusen. In addition, as the yeast flocculates out it will carry with it some of the isomerized bittering compounds.
While there are many published sources that provide specific figures for each of these factors along with involved calculations, utilization percentage can be estimated fairly well. When doing a full-wort boil of moderate gravity (1.045 to 1.055), with hops boiled for 60 minutes or more, assume a utilization of 30 percent. (See Hop Utilization Percent)
You can make a pretty good estimate of how your own brewing conditions will affect utilization and modify the numbers accordingly. For example if you are unable to do a full-wort boil or the beer you are making is of a relatively strong gravity, then it would probably be safe to assume that the estimate can be factored down by 2 percent to 3 percent.
Go back over your previous recipe records and convert your hop amounts to IBUs. By doing this you can compare what your tastebuds tell you about your latest batch of beer to the published standards for the appropriate styles and begin to develop your own figures with respect to your brewing style and equipment. As this information continues to build, batch after batch, it will become easy to convert among recipes. Multiple hop additions will become easy to calculate by doing a separate IBU calculation for each hop addition and adding together the results to obtain a total bitterness. By experimenting with these numbers, a brewer need never worry about having an over- or under-bittered batch of homebrew again.
Standard IBU Formula
IBU = weight of hops in ounces x alpha acid % x utilization % volume of final batch in gallons x 1.34*
*constant to convert measurement into US standards
1 oz. of Northern Brewer (10% alpha acid) boiled for 60 minutes in a full-wort boil of moderate gravity wort for a five-gallon batch.
IBU = 1 x 10 x 30 / 5 x 1.34 = 44.77
Let’s say you are using last year’s recipe but the alpha acid of your hops has changed to 8% alpha acid. You will need to solve for the weight of hops with the new alpha acid percent. Rearrange the formula so that...
weight of hops in ounces =
IBUs desired x volume of final batch in gallons x 1.34* / alpha acid percent x utilization percent
*constant to convert measurement into US standards
oz. hops = 44.77 x 5 x 1.34 / 8 x 30 = 1.25 oz. hops
Why AAUs/HBUs are Helpful
The measurement of Alpha Acid Units, also called Homebrew Bitterness Units, gives the potential bitterness of hops by multiplying the amount of hops used by the hops’ percent alpha acid.
AAUs = % alpha x oz.
This number is not useful in determining the bitterness of a beer because it does not take into account such factors as length of boil, gravity of wort, and volume of boil. These (and many other) factors affect the bitterness of a beer. These factors are addressed more closely in the IBU calculation (though still not completely).
AAUs/HBUs are useful as a quick reference when you need to convert a recipe for hops with different alpha acid percentages. If the recipe calls for 2 oz. Northern Brewer with 8 percent alpha acid but your Northern Brewer is 10 percent alpha acid, you can use the AAU calculation to keep your IBUs the same.
For example: last year’s recipe equaled 16 AAUs (8% alpha x 2 oz.). Reverse the AAU formula to solve for ounces:
oz. = AAUs/% alpha.
For this recipe,
oz. = 16 AAUs/10% alpha
You would need to use 1.6 oz. for the Northern Brewer.